Temperature-controlled dimensionality variety from 3D to 2D and 1D based on Cd(II)/ip/bmb polymers

Article abstract of DOI:10.1016/j.inoche.2011.04.018

By controlling the hydrothermal reaction temperature, three coordination polymers with the same ingredients but different dimensional architectures from 3D to 2D and 1D have been obtained, namely, [Cd(bmb)(Hip)₂] _n (1), {[Cd(bmb)(ip)]?H₂O}_n (2), and [Cd(bmb)(ip)(CH₃CH₂OH)(H₂O)]_n (3) (bmb = (1,4-bis(2-methylbenzimidazol-1-ylmethyl) benzene); H₂ip = 1,3-benzenedicarboxylic acid). A careful investigation of the structures discloses that temperature parameters have significant effects on the conformations and coordination modes of the ligands and enhanced temperature favors the formation of higher dimensional products. In addition, the thermal stabilities and photoluminescence properties of polymers 1-3 in the solid state have also been investigated.

Full text of DOI:10.1016/j.inoche.2011.04.018

Inorganic Chemistry Communications 14 (2011) 1204–1208
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Temperature-controlled dimensionality variety from 3D to 2D and 1D based on
Cd(II)/ip/bmb polymers
⁎
Chunying Xu, Linke Li, Qianqian Guo, Hongwei Hou , Yaoting Fan
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
By controlling the hydrothermal reaction temperature, three coordination polymers with the same ingredients
but different dimensional architectures from 3D to 2D and 1D have been obtained, namely, [Cd(bmb)(Hip)2]n (1),
{[Cd(bmb)(ip)]·H2O}n (2), and [Cd(bmb)(ip)(CH3CH2OH)(H2O)]n (3) (bmb=(1,4-bis(2-methylbenzimidazol-
1-ylmethyl) benzene); H2ip=1,3-benzenedicarboxylic acid). A careful investigation of the structures discloses
that temperature parameters have significant effects on the conformations and coordination modes of the ligands
and enhanced temperature favors the formation of higher dimensional products. In addition, the thermal
stabilities and photoluminescence properties of polymers 1–3 in the solid state have also been investigated.
© 2011 Elsevier B.V. All rights reserved.
Received 25 February 2011
Accepted 15 April 2011
Available online 23 April 2011
Keywords:
Temperature
Dimensionality
Cadmium
Photoluminescence
Recently, the rational design and construction of metal–organic
coordination complexes with the same ingredients but different
structures have attracted wide-spread interest due to their potential
applications in magnetism, adsorption, separation, transportation of
gasses and liquids as well as catalysis [1–4]. Remarkable progress has
been made on the theoretical analysis and practical manipulation of
such interesting complexes, but the dimensionality control of the
resulting products is still a great challenge at present [5–7], because the
final structures are frequently modulated by various factors [8–18].
Among them, one factor, so-called reaction temperature, can't be
ignored based on the following considerations: i) under different
temperatures, flexible organic ligands inherently hold the potential of
adopting different conformations [19,20]; ii) the reaction temperature
plays a crucial role in tuning the coordination mode of organic
carboxylic ligand [21,22]. So, under the same reagent conditions, the
reaction temperature can be used as a structure-directing agent and
various dimensional (0D–3D) architectures, to some extent, could be
purposefully obtained. However, due to the complexity and difficult
prediction of the final composition and structure, the influential
principles are still less ascertained [23–25]. Consequently, the explora-
tion of tuning the dimensionality of a complex through reaction
temperature modification provides an impetus for the further research
of coordination complexes. In this work, the dimensionality of Cd(II)/ip/
bmb polymers were adjusted from 3D self-penetrated pillared-layer
framework to 2D (4, 4) grid-layer and 1D trapezoid-like chain by
varying the hydrothermal reaction temperatures. Structural determi-
nations indicates that the mixed ligands with different conformations
and coordination modes compete to bind toward the octahedral Cd(II)
ion, which changes the spatial arrangement of the mixed ligands and
tunes the dimensionality of the resulting complexes to reduce with the
decreasing of reaction temperature.
As shown in Scheme 1, the freely conformational bmb, various
coordination modes of H2ip, and cadmium with high coordination
number are selected to synthesis polymers 1–3 [26] under different
hydrothermal conditions. At a high temperature of 160 °C, octahedral
Cd(II) center is ligated by two trans-conformational bmb (with a
Ndonor⋯N–Csp3⋯Csp3 torsion angle of 77.26°) and four Hip− with the
coordination mode of type I (Scheme 2). As a result, a charming 3D
160 °C
H2O
3D (1)
Cd(II)
+
bmb
+
130 °C
H2O
2D (2)
1D (3)
H2ip
100 °C
EtOH/H2O
⁎
Corresponding author. Tel./fax: +86 371 67761744.
E-mail address: houhongw@zzu.edu.cn (H. Hou).
Scheme 1. Schematic diagram of the synthetic conditions and topology for polymers 1–3.
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.04.018

C. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1204–1208
1205
torsion angle of 87.95°) and two ip2− (with the coordination mode of
type III) only occupy three coordinated sites of octahedral Cd(II), and
the others are occupied by terminally coordinated solvent molecules.
Thus, it can be concluded that the different reaction temperatures
essentially influence the conformation of bmb and coordination mode
of ip2−, and induce different cooperative coordination fashions of the
two ligands with the octahedral Cd(II) ion, which leads the
dimensionality of polymers 1–3 to reduce with the decreasing of
reaction temperature.
O
OH
O
O
O
O
M
M
M
O
O
O
O
O
O
M
M
M
(I)
(II)
(III)
Scheme 2. The coordinated mode of the H2ip ligand.
Single crystal X-ray diffraction analysis [27] indicates that each Cd(II)
ion of 1 exhibits a slightly distorted octahedral environment (Fig. 1a),
which comprises four carboxylic oxygen atoms from two symmetry-
related Hip− anions in the equatorial plane and two nitrogen atoms
from two bmb ligands in the apical site. The incompletely deprotonated
Hip− ligands link adjacent Cd(II) ions to form a 2D (4,4) coordination
network in bc-plane (Fig. 1b). Obviously, due to the specific spatial
torsion of Hip− (with a Odonor⋯Csp2⋯Csp2⋯Odonor torsion angle of
155.92°), the charming right and left handed helical chains with an
alternate arrangement fashion are observed in Cd(II)/(Hip) layer. The
helical pitches are both 11.070(2) Å corresponding to the length of b-axis.
Furthermore, these helical layers are pillared by trans-conformational
bmb via the Cd–N connections to generate a 3D pillar-layered framework
self-penetrated pillared-layer framework 1 is obtained. Reducing the
reaction temperature to 130 °C, bmb also adopts trans-conformation
but with a larger Ndonor⋯N–Csp3⋯Csp3 torsion angle of 99.87°, which
occupies two coordinated sites of octahedral Cd(II) ion. Two ip2− with
the chelating coordination fashion of type II (Scheme 2) occupy the
other four coordinated sites of octahedral Cd(II) ion. Under the
cooperation of bmb and ip2−, 2 exhibits 2D (4, 4) grid-layers. When a
low hydrothermal reaction temperature of 100 °C is selected, a 1D
trapezoid-like chain 3 is generated. Different from 1 and 2, an EtOH/
H2O mixed solution is used due to the poor solubility of bmb at low
temperature. The formation of low-dimensional polymer may be due
to the fact that trans-conformational bmb (with a Ndonor⋯N–Csp3⋯Csp3
Fig. 1. (a) Coordination environment of Cd(II) ion in polymer 1 with hydrogen atoms omitted for clarity (thermal ellipsoids at 30% probability level). (b) The Hip− links adjacent
Cd(II) ions to form a 2D (4,4) coordination network in bc plane with the charming right and left handed helical chains arranging alternately. (c) The 2D layers are pillared by bmb to
generate a 3D pillar-layered framework.

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C. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1204–1208
(Fig. 1c). To further understand the structure of 1, the Cd(II) atom can be
considered as a six-connected node, and the bmb and Hip− serve as
linkers. As highlighted in Scheme 1, the combination of nodes and linkers
produces a 6-connected framework with a Schläfli symbol of 44·610·8.
Careful inspection of such framework suggests that the net is self-
penetrating. Each six-membered shortest circuit is catenated by two rods
of the same network. Self-penetration is an unusual form of topological
entanglement [28–30]. Thus, 1 can be considered as another example of
the self-penetrated coordination polymers.
In 2, the coordination environment around the Cd(II) center is best
portrayed as a distorted [CdO4N2] octahedral geometry (Fig. 2a), ligated
by four oxygen atoms (O1, O2, O1B, and O2B) from two symmetry-
related ip2− anions and two nitrogen atoms (N1 and N1B) from two
distinct bmb. The bmb adopts trans-conformation with two flexures
toward opposite direction, and links neighboring Cd(II) ions to afford a
fascinating meso-helical chain with left- and right-handed helical loops
in one single strand along the c-axis (Fig. 2b). The ip2− anion takes the
coordination mode of type II (as illustrated in Scheme 2) bridging
adjacent Cd-bmb meso-helices (Fig. 2c) to generate a 2D puckered layer.
The Cd···Cd distance across ip2− is 9.267 Å. Simplifying the Cd(II) as a
node, polymer 2 shows a 44 topology. These 2D grid-layers are stacking
in an offset fashion along ac- plane with some displacement between
two layers (Scheme 1).
The crystal structure of 3 also contains an octahedral Cd(II) ion,
which is coordinated by three oxygen atoms from different carboxylic
groups of two symmetry-related ip2− anions, one nitrogen atoms from
bmb, and two oxygen atoms from water and ethanol molecules.
Different from polymers 1 and 2, two carboxylic groups of ip2− adopt
different coordination modes: one carboxylic group adopts bidentate
chelate mode and the other adopts monodentate fashion (type III in
Scheme 2). The ip2− anion acts as μ2-bridge linking Cd(II) ions to
produce a 1D linear chain. In addition, the adjacent Cd/ip chains are
Fig. 2. (a) Coordination environment of Cd(II) ion in polymer 2 with hydrogen atoms and free water molecules omitted for clarity (thermal ellipsoids at 30% probability level).
(b) The bmb linking neighboring Cd(II) ions afford a fascinating meso-helical chain with left- and right-handed helical loops in one single strand along the c-axis. (c) The ip2− anions
bridge adjacent Cd-bmb meso-helices to generate a 2D layer.

C. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1204–1208
1207
of 3.84% is observed from 314 °C to 355 °C corresponding to the loss of
a coordinated water molecule (calcd: 3.44%). The unusual weight loss
may be because the presence of hydrogen-bonding between the
coordinated water molecules and ip2− anions [31]. The removal of
organic components occurs in the range of 381–541 °C. In addition,
polymers 1–3 show different fluorescent emissions bands at 412 nm
for 1 (λex =360 nm), 445 nm for 2 (λex =370 nm), and 411 nm for 3
(λex =354 nm) respectively (Fig. S3a). Similar emissions can also be
observed for free H2ip (λem =389 nm, λex =321 nm) (Fig. 4). Thus,
the emissions of the three polymers should be assigned to intra-ligand
charge transitions of H2ip. The red-shift is probably caused by the
coordination of ip2− to the metal centers [32]. The varying degrees of
red-shift are due to the changes of the coordination environments and
various coordination mode of H2ip. The decreasement of fluorescent
intensities of polymers 1–3 is attributed to the coordination of ip2− to
the metal center, which lowers the rigidity of ip2− and increases the
loss of energy by radiationless decay [33]. Moreover, the ip2− ligands
adopt different coordination modes in polymers 1–3 (Scheme 2) and
the rigid of ip2− with bidentate chelating coordination fashion of
carboxylic group is stronger than that of ip2− with monodentate
coordination mode of carboxylic group. So, the fluorescent intensities
of 2 and 3 are stronger than that of 1.
In summary, three high thermal stability fluorescent polymers
with the same components, 3D self-penetrating pillared-layer
framework for 1, 2D (4, 4) grid-layer for 2, and 1D trapezoid-like
chain for 3 have been obtained at the hydrothermal reaction
temperatures of 160 °C, 130 °C and 100 °C respectively. Further
investigation indicates that the conformations and coordination
modes of the ligands in 1, 2 and 3 are obviously different. And this
difference induced by varying reaction temperatures should be
responsible for the fact that the overall dimensionality of the resulting
complex reduces with the decreasing of reaction temperature.
Fig. 3. The center Cd(II) ion show a distorted octahedral geometry in polymer 3. The
adjacent Cd/ip chains are joined by bmb to form a slightly distorted 1D trapezoid-like
chain decorated with water and ethanol molecules at both sides.
joined by trans-conformational bmb to form a slightly distorted 1D
trapezoid-like chain decorated with water and ethanol molecules at
both sides (Fig. 3).
In order to study the differences between these complexes with
the same ingredients but different dimensionalities in the properties,
thermogravimetric analyses (TGA) and photoluminescence spectrum
of polymers 1–3 were performed. The PXRD patterns were compa-
rable to the corresponding simulated ones calculated from the single-
crystal diffraction data (Fig. S1 in the Supplementary information),
indicating a pure phase of each bulky sample. As shown in Fig. S2 (in
the Supplementary information), polymers 1–3 show high thermal
stabilities. For the 3D framework 1 without any lattice molecules, only
one weight-loss stage is observed between 365 °C and 682 °C, which
is attributed to the collapse of the pillared-layer framework (obsd:
85.8%; calcd: 86.1%). For the 2D polymer 2, a weight loss of 2.31% is
detected in the range of 46–131 °C corresponding to the loss of lattice
water molecule (calcd: 2.73%), and then a plateau region follows. The
overall framework starts to decompose at about 385 °C. The TGA
curve of low-dimensional polymer 3 displays a multi-step thermal
weight-loss process. The first step weight loss (obsd: 7.99%) at 130–
171 °C, corresponding to the loss of an ethanol molecule (calcd:
8.79%), and then a plateau region follows. The second step weight loss
Acknowledgments
This work was financially supported by the National Natural
Science Foundation (Nos. 20971110, 91022013 and 20801048),
Program for New Century Excellent Talents of the Ministry of
Education of China (NCET-07-0765), and the Outstanding Talented
Persons Foundation of Henan Province.
Fig. 4. Photoluminescences of polymers 1–3 and free ligands.

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C. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1204–1208
[26] (a) Synthesis of 1: mixture of Cd(NO3)·4H2O (61.7 mg, 0.2 mmol), bmb
A
Appendix A. Supplementary material
(36.6 mg, 0.1 mmol), isophthalic acid (33.2 mg, 0.2 mmol) and NaOH (8.0 mg,
0.2 mmol) in 10 mL distilled H2O was sealed in a 25 mL Teflon-lined stainless
steel container and heated at 160 °C for 3 days. After the mixture cooled to room
temperature at a rate of 5 °C/h, colorless pillar-like crystals of 1 were obtained
with a yield of 32% (based on Cd). Anal. Calc. For C40H32N4O8Cd (%): C, 59.38; H,
3.99; N, 6.92. Found: C, 59.86; H, 4.13; N, 7.11. IR (KBr, cm−1): 2924(m), 2853(w),
1707(w), 1609(m), 1555(s), 1508(w), 1457(m), 1382(s), 1226(w), 1163(w),
1016(w), 990(w), 916(w), 857(w), 745(s), 719(m), 662(w), 475(w);
(b) Synthesis of 2: The reaction of 2 was carried out in the procedures similar to
those of 1, only heating to 130 °C. Colorless prism crystals were generated directly
with a yield of 52% (based on Cd). Anal. Calc. For C32H28N4O5Cd (%): C, 58.15; H,
4.27; N, 8.48. Found: C, 57.98; H, 4.04; N, 8.62. IR (KBr, cm−1): 3435(s), 2923(w),
2166(w), 1607(s), 1557(s), 1506(w), 1477(w), 1458(m), 1406(m), 1379(m),
1292(w), 1162(m), 1105(m), 991(m), 918(m), 860(m), 735(w), 719(s), 659(w),
615(w); (c) Synthesis of 3: The synthetic procedure for 3 is similar to that for 1
except that H2O was replaced by H2O/CH3CH2OH (4:1) and the hydrothermal
temperature was 100 °C, affording colorless plate-like crystals with a yield of 59%
(based on Cd). Anal. Calc. For C22H23N2O6Cd (%): C, 50.44; H, 4.23; N, 5.35. Found:
C, 50.71; H, 4.14; N, 5.57. IR (KBr, cm−1): 3399(s), 2973(w), 2370(w), 1633(m),
2603(s),1550(s), 1510(m), 1474(m), 1458(m), 1382(s), 1291(m), 1159(m), 1076(w),
1012(w), 931(w), 838(m), 808(w), 742(s), 725(s), 670(w), 612(w).
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center, CCDC
reference numbers 813926-813928 for complexes 1–3. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.
html (or from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033). Details on
XRD patterns, TGA curves and tables are list in supplementary
content. Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.inoche.2011.04.018.
References
[1] Z. Zeng, R.-H. Wang, B. Twamley, D.A. Parrish, J.M. Shreeve, Chem. Mater. 20
(2008) 6176.
[2] M. Kurmoo, H. Kumagai, M. Akita-Tanaka, K. Inoue, S. Takagi, Inorg. Chem. 45
(2006) 1627.
[27] Crystal data for 1: C40H32N4O8Cd, M=809.11, monoclinic, space group P2(1)/c,
a=9.6181(19), b=11.070(2), c=16.659(3) Å, β=101.39(3), v=1738.7(6) Å3,
Z=2, Dc=1.545 cm−3, F000=824.0, Mo Kα radition, λ=0.71073 Å, T=293 (2)
K, μ=0.690 cm−1, GOF=1.076, R1 =0.0289, wR2 =0.0663 [IN2σ(I)]. Adsorption
correction: Multi-scan (Tmin/Tmax = 0.883/0.914). Crystal data for 2:
C32H28N4O5Cd, M=661.00, monoclinic, space group C2/c, a=9.2668(19),
b=9.2668(19), c=17.267(3) Å, β=101.49(3), v=2952.3(10) Å3, Z=4,
Dc=1.483 cm−3, F000=1336.0, Mo Kα radition, λ=0.71073 Å, T=293 (2) K,
μ=0.787 cm−1, GOF=1.080, R1 =0.0525, wR2 =0.1053 [IN2σ(I)]. Adsorption
correction: Multi-scan (Tmin/Tmax = 0.797/0.834). Crystal data for 3:
[3] K. Liu, X. Zhu, J. Wang, B. Li, Y. Zhang, Inorg. Chem. Commun. 13 (2011) 976.
[4] J.-K. Cheng, J. Zhang, P.-X. Yin, Q.-P. Lin, Z.-J. Li, Y.-G. Yao, Inorg. Chem. 48 (2009)
9992.
[5] E.-C. Yang, T.-Y. Liu, Q. Wang, X.-J. Zhao, Inorg. Chem. Commun. 14 (2011) 285.
[6] J.J.M. Amoore, C.A. Black, L.R. Hanton, M.D. Spicer, Cryst. Growth Des. 5 (2005)
1255.
[7] V. Štrukil, L. Fábián, D.G. Reid, M.J. Duer, G.J. Jackson, M. Eckert-Maksić, T. Friščić,
Chem. Commun. 46 (2010) 9191.
[8] W.-C. Song, J.-R. Li, P.-C. Song, Y. Tao, Q. Yu, X.-L. Tong, X.-H. Bu, Inorg. Chem. 48
(2009) 3792.
C
22H23N2O6Cd, M=523.84, triclinic, space group P-1, a=10.187(2), b=10.874
[9] Q. Zhang, J. Zhang, Q.-Y. Yu, M. Pan, C.-Y. Su, Cryst. Growth Des. 10 (2010) 4076.
[10] M. Yoshizawa, M. Nagao, K. Umemoto, K. Biradha, M. Fujita, S. Sakamoto, K.
Yamaguchi, Chem. Commun. (2003) 1808.
[11] S.R. Batten, K.S. Murray, Coord. Chem. Rev. 246 (2003) 103.
[12] J. Xu, X.-Q. Yao, L.-F. Huang, Y.-Z. Lia, H.-G. Zheng, CrystEngComm 13 (2011) 857.
[13] S.M. Chen, C.Z. Lu, Q.Z. Zhang, J.H. Liu, X.Y. Wu, Eur. J. Inorg. Chem. (2005) 423.
[14] S.H. Cho, B. Ma, S.T. Nguyen, J.T. Hupp, T.E. Albrecht-Schmitt, Chem. Commun.
(2006) 2563.
[15] Q.-K. Liu, J.-P. Ma, Y.-B. Dong, J. Am. Chem. Soc. 132 (2010) 7005.
[16] D.P. Martin, R.M. Supkowski, R.L. LaDuca, Inorg. Chem. 46 (2007) 7917.
[17] B. Zheng, H. Dong, J. Bai, Y. Li, S. Li, M. Scheer, J. Am. Chem. Soc. 130 (2008) 7778.
[18] J.-G. Lin, Y.-Y. Xu, L. Qiu, S.-Q. Zang, C.-S. Lu, C.-Y. Duan, Y.-Z. Li, S. Gao, Q.-J. Meng,
Chem. Commun. (2008) 2659.
(2), c=11.291(2) Å, α=64.42(3), β=71.62(3), γ=75.04(3), v=1059.7(3) Å3,
Z=2, Dc=1.635 cm−3, F000=526.0, Mo Kα radition, λ=0.71073 Å, T=293 (2)
K, μ=1.072 cm−1, GOF=1.081, R1 =0.0312, wR2 =0.0740 [IN2σ(I)]. Adsorption
correction: Multi-scan (Tmin/Tmax =0.764/0.851). X-ray data collection was
carried out using
a Rigaku Saturn 724 CCD diffractometer. The data were
corrected for Lorentz and polarization effects. The structures were solved by
direct methods and refined with a full-matrix least-squares technique based on F2
with the SHELXL-97 crystallographic software package. The hydrogen atoms were
assigned with common isotropic displacement factors and included in the final
refinement by using geometrical restrains.
[28] S.S. Chen, J. Fan, T. Okamura, M.S. Chen, Z. Su, W.Y. Sun, N. Ueyama, Cryst. Growth
Des. 10 (2010) 812.
[29] G.S. Yang, Y.Q. Lan, H.Y. Zang, K.Z. Shao, X.L. Wang, Z.M. Su, C.J. Jiang, Cryst. Eng.
Commun. 11 (2009) 274.
[30] V.A. Blatov, L. Carlucci, G. Ciani, D.M. Proserpio, CrystEngComm 6 (2004) 377.
[31] W.-L. Zhang, Y.-Y. Liu, J.-F. Ma, H. Jiang, J. Yang, G.-J. Ping, Cryst. Growth Des.
8 (2008) 1250.
[32] K. Jiang, L.-F. Ma, X.-Y. Sun, L.-Y. Wang, CrystEngComm 13 (2011) 330.
[33] G.-X. Liu, K. Zhu, H.-M. Xu, S. Nishihara, R.-Y. Huang, X.-M. Renac, CrystEngComm
12 (2010) 1175.
[19] V. Iancu, A. Deshpande, S.-W. Hia, Nano Lett. 6 (2006) 820.
[20] F. Luo, M.-b. Luo, Y.-h. Liu, CrystEngComm 12 (2010) 1750.
[21] D. Liu, Z.-G. Ren, H.-X. Li, Y. Chen, J. Wang, Y. Zhang, J.-P. Lang, Cryst. Eng.
Commun. 12 (2010) 1912.
[22] J.-D. Lin, J.-W. Cheng, S.-W. Du, Cryst. Growth Des. 8 (2008) 3348.
[23] M.-L. Tong, S. Kitagawa, H.-C. Chang, M. Ohba, Chem. Commun. (2004) 418.
[24] X. Wang, L. Liu, T. Makarenko, A.n J. Jacobson, Cryst. Growth Des. 10 (2010) 3752.
[25] S. Dey, B. Ojha, G. Das, CrystEngComm 13 (2011) 269.